The present invention is related to a filter medium. More specifically, the present invention is related to a filter medium for gaseous fluids.
Filter media having large interfiber pores and, thus, a high permeability typically contain sparsely packed relatively thick fibers. Such filter media require relatively low driving pressure to provide adequate filtration throughput rate and extended service-life. However, highly permeable filter media, e.g., residential glass fiber HVAC filters, only provide a low filtration efficiency in that the large interfiber pore structures of the media do not have interstitial configurations that are suitable for entrapping fine contaminant particles. Consequently, coarse fiber filter media have not been used in fine particle filtration applications.
In contrast, microfiber nonwoven webs, such as meltblown fiber webs, have been used as fine particle filter media. The densely packed fine fibers of these webs provide fine interfiber pore structures that are highly suitable for mechanically trapping or screening fine particles. However, the fine pore structure of meltblown fiber webs and other similar microfiber webs that have densely packed fine fibers results in a low permeability, creating a high pressure drop across the webs. Consequently, the low permeability of fine fiber filter media requires a high driving pressure to establish an adequate filtration throughput rate. Furthermore, as contaminants accumulate on the surface of the filter media, the contaminants quickly clog the small interfiber pores and further reduce the permeability of the media, thereby even further increasing the pressure drop across the media and rapidly shortening the service-life.
Additionally, microfiber web filter media do not tend to have a physical integrity that is sufficient enough to be self-supporting. Although the physical integrity of microfiber filter media can be improved by increasing the basis weight or thickness thereof, the increased basis weight or thickness exacerbates the pressure drop across the filter media. As such, microfiber web filter media are typically laminated to a supporting layer or fitted in a rigid frame. However, the conventional supporting layer or rigid frame does not typically contribute to the filtration process and only increases the production cost of the filter media.
There remains a need for self-supporting filter media that provide combinations of desirable filter properties, including high filtration efficiency, high permeability, low pressure drop, high throughput rate and long service-life.